Collection of “trace analytes” or “target analytes” may provide early warning of security risks or material information about environmental conditions generally. A preferred class of target analytes includes particulates and vapors, such as volatiles associated with explosives, where the volatiles are frequently found adsorbed on and associated with particles. As will be readily appreciated, particles adherent to substrates may be dislodged and aerosolized by application of high velocity jets to a substrate surface from a distance and are thus subject to analysis. The capacity to detect hazardous residues associated with particles on objects or vehicles, for example, and on persons with a history of having handled hazardous materials, is a rising security concern.
While it is known that an impinging directional jet may be used to dislodge and aerosolize particles and vapors from substrates, the jet action also scatters the particles, essentially diluting them in a larger sample volume and propelling them from the sampling area. While suction ports have been devised to collect larger sample volumes, the capacity to efficiently collect the dislodged material deteriorates rapidly with the distance of the suction port from the sample and the linear vector of the jet burst relative to the substrate or suction port. In short, some or all of the sample may be propelled away from the suction intake, leading to missed detection. While the behavior of the jet is reasonably predictable for a flat surface, when the surface is tilted away from the suction port or includes surface irregularities, significant sample loss occurs. This is particularly true for axisymmetric jets. Any increase in sample volume decreases sensitivity, and thus a satisfactory solution to this problem has not been demonstrated.
Related art is described for example in U.S. Pat. Nos. 5,491,337, 6,708,572 and 6,642,513. Various systems for hand search, gate security, walkthrough portals, and vehicle sampling have been advanced, and generally involve some kind of contacting sample collection such as a swab, or by concentration of very, very large volumes of air, such as from a portal or enclosure with a large fan-driven collection ductwork.
However, non-contact, open “sniffing” may be preferred, where air and particles associated with a suspect package or person are drawn into a nose-like detection system termed here a “non-contacting sampler”. Early systems include those described in U.S. Pat. Nos. 5,491,337, 4,909,090, 7,275,453, 6,887,710, 7,942,033, 6,345,545, 7,299,711, 6,978,657, 6,604,406, 6,085,601, 5,854,431, RE38,797, 5,465,607, 4,987,767, 7,299,710, 8,307,723, 8,475,577 and in U.S. Pat. Publ. Nos. 20110186436 and 2012/0105839.
In preliminary work described in U.S. Pat. Publ. No. 20110186436 to Novosselov, continuous, non-contact sampling of trace materials from level surfaces was demonstrated for up to 6 inches distance using a circular jet array in which the jets define a “curtain wall” of moving air around a suction port, which is in turn surrounded by a secondary jet wall. For a given particle size, the drag force acting on the particle is related to the velocity of the wall jet at the location of the particle in relation to the jet impingement point. The higher the velocity of the wall jet at the particle location, the greater the lateral force for particle mobilization. But, performance limitations were reached. There are factors that limit the performance of the circular jet array sampler that cannot readily be overcome:
A) Limits on wall jet properties: The force applied to a particle on a surface greatly diminishes with an increase in the standoff distance H. With spaced axisymmetric jets, the jet spreads in all directions as a free jet and loses its momentum. Furthermore, after impingement on a solid surface, the wall jet velocity from the axisymmetric nozzle is rapidly diminished (due to momentum expansion) at shorter distances from the impingement point according to the r2 law. The removal area (maximum wall shear stress location) for axisymmetric jet is relatively small and limits the interrogation zone of the circular array sampler. The force applied to the particle on a surface diminishes by a negative exponent of the standoff distance.
B) Geometric limits: Jets converge on a single point in the best designs, making collection of the mobilized sample impossible once the convergence distance is reached. Coherent jet momentum capable of imparting sufficient wall jet velocity has not proved possible at standoff distances greater than six inches for round nozzles because of parasitic losses in the free jet momentum. Circular jet geometry is more susceptible to the surface inclination and imperfections, as the individual jet timing and any sampler head misalignment will cause particle dispersion.
C) Limitations on independent jet function: Circular jet orifices were not readily controlled so as to vary timing and angulation. Pulse delay resulted in openings in the curtain wall through which particles were lost. Complex manifold geometry associated with circular jet arrays and uneven pressure distribution upstream of the nozzles results in the difficulties with controlling timing and dynamic pressure of each independent jet in the array to adjust for variations in stand-off distances, surface inclination and surface morphology.
In short, detection at a non-contact standoff of nine or twelve inches has not been possible using conventional technologies. The jet geometries developed to date are not robust and have not offered a solution to the problem of sampling from moving and complex surface geometries. Roadway detection of improvised explosive devices is needed, for example. The sampling problem is exacerbated where the substrate is an uneven surface, such as under a vehicle and the vehicle, with sampling apparatus attached, is in motion. Similarly, inspecting vehicles for concealed explosives is made difficult by the complex surfaces inside and on the underside of the vehicle.
A solution to these interrelated problems has not been achieved by trial and error or by computational fluid dynamics. Among other issues, special treatment for turbulence is required to obtain solutions. Even so, turbulence modeling techniques are not always consistent with the experimental data. Methods include Reynolds-averaged Navier-Stokes (RANS) models, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) are challenging due to prohibitively large grid requirement near the wall, especially for complex, real world sampling scenarios.
However, new computational approaches have been needed to speed directions useful in guiding experimental confirmation. To date, no fully operational trace analyte sampling and detection system for high throughput operation at larger standoff distances has been achieved. Any detection system is only as sensitive as the front-end sampling system. Thus, there is a need in the art, for a trace analyte surface sampling apparatus or system that overcomes the problems and limitations in the art, of which the above described literature is generally representative.